The Virtues of the
Diffusion Screen in the 35mm Slide Copier/Film Scanner

And its Parallels in
Brightfield Microscopy

By Paul James

The practice of
siting a diffusion screen at the front of the slide 35mm copier
I'd imagined was to ensure the copying device's successful
imaging in most of the usual lighting situations that
photographers would have at their disposal, for it conveniently
broadens the illuminant's field considerably by default and
dispenses with the need to rely on purpose made illumination
units that would only increase costs. Despite its simplicity it
works very effectively. The only proviso being that the colour
correction/white balance is taken into account when capturing the
image on the digital camera's sensor from any one of a variety of
light sources that users might take advantage of.

Fortuitously, the
diffusion screen in the 35mm copier not only broadens the field
of illumination sufficiently to flood the entire 35mm
format......its principal purpose, but it also manages to
severely reduce any high contrast diffraction which would have
been raised in profusion by all the hard edged unwanted
particulates and markings that might litter or are embedded in an
old film emulsion. Conducting an A-B test with and without the
screen in place soon reveals its effectiveness in this
situation, though the relative differences between the 2 types of
illumination depends upon the alternative light source used
besides the diffusion screen. A near parallel light source will
reveal every speck across the emulsion and film base with great
clarity. Of course the more robust particles and marks cannot be
drowned out from the image by diffused lighting, but the greater
proportion of the unwanted finer dross can be dispensed with,
which is remarkable given the utter simplicity of the situation.

The All
Important Illumination........Taking the technique to extremes.

I thought
I'd show as a classic example of how the process of imaging by 2
entirely different lighting techniques can produce 2 radically
different effects. An enlarged crop ( Photomacroscope) from a
Kodachrome colour slide taken in 1954 which is in poor condition
is first shown illuminated with partially oblique lighting
( ie a near parallel source tilted to one side ). The depiction
of detail is severely emphasised, especially of grain structure,
particulates and other markings such as scratches are highlighted
very effectively. Diffraction artifacts and colour distortion and
entirely unsuitable in this situation :-

The second image
above was raised by near parallel lighting in line with the
optical axis of the macroscope but with a plastic diffuser in
contact with its emulsion layer. Thus the light rays
emerging through the diffuser would be randomly deviated through
a very wide angle approaching 180 degrees, though the bulk would
be less than this. Note the virtual absence of markings and fine
debris across the emulsion. Despite the severity of flooding the
film emulsion with chaotically disorganised diffused light, the
image remains intact and visually comprehensible.

The abundance of
'detail' in the first image is overly exaggerated by oblique
lighting, and though it is a most useful lighting technique in
microscopy for that very reason, its ruthless exposure of every
nuance of detail simply spoils the more subjective elements of
the image. The opposite extreme shown in the diffuser image
efficiently rids the subject of unnecessary distractions of fine
detail induced diffraction rather dramatically, though it can't
remove major defects near centre bottom and others of less
importance. Despite the diffuser's apparent detail and contrast
moderating properties on film emulsions, the overall effect is
very satisfying especially at normal viewing scales.

B&W
Negative Treatment

Above left is shown
a small crop of a B&W negative which is about 45 years old,
with fungal hyphae infestation. It was illuminated by near
parallel light only. The image right taken with the diffuser
screen was in contact with the emulsion using the same
illuminant. Again a dramatic reduction of unwanted artifacts. The
grain's acutance seems to have been slightly muted too, but I
think this can be a positive benefit in the presentation of
portraiture.

The image left
appears crisp in comparison to that shown on the right. But it
must be remembered that a large proportion of the 'detail' in the
image shown left is actually composed of diffractive anomalies
from both fungal hyphae as well as the silver grain particles.
Thus the diffraction bereft image right appears soft by
comparison. The underlying image remains similar in both
examples.

These intimate
views of the film's surface were imaged through the
photomacroscope. I used it because its slightly higher
amplification of the film reveals the differences more easily
onscreen. The bright field illumination was generated from a
single LED suitably modified for macro fields by a lamphouse
condenser, and behaved as a near parallel source. The images are
straight out of the camera ( Sony NEX 3 body ) and cropped for
convenient onscreen illustration.

In the darkroom
situation, these imperfect negatives respond to a diffuser screen
in the enlarger in similar fashion.

Parallels
In Bright Field Compound Microscope Practice

The reality of what
is going on within the slide copier is loosely comparable with
the illumination techniques of Bright Field microscopy. The
observation of a semi-transparent specimen such as a film
negative/positive whilst opening and closing the substage
condenser's iris diaphragm will reveal a similar range of
suppression of diffractionally orientated detailing. The wider
the condenser's aperture the greater the suppression of the
specimen's semi-transparent structure : Conversely as the iris
closes down, the pencil beam passing through the specimen is much
nearer a parallel beam and exacerbates the presence of
diffraction patterns of minute particulate or linear form
greatly. The skill in brightfield microscopy is in developing an
awareness of the importance of the setting of the substage
condenser's iris diaphragm for a given specimen. Yet despite this
well understood imaging principle, the use of diffused lighting
for certain subjects that are of a more robust nature such as
insect parts etc., greatly enhances an otherwise overly contrasty
image that conventional brightfield tends to raise in this
situation. The diffuser can, if suitably close to the under
surface of the slide generate moderately high NA light and
obliterate virtually all the diffractive components in one
process. So a fair amount of potential resolution is maintained,
but contrast is muted, but not too severely. Since the colour
slide/B&W negative's base emulsion detail is more robust than
many a microscope specimen's delicate semi transparent structure,
the muting of contrast that diffusers can cause does not change
the image information to any significant degree.

Proximity
of the Diffusing Screen in the Slide Copier

The
diffusing screen's efficiency at removing unwanted dross and
markings is at its greatest when in virtual contact with the
film's emulsion. But this requires that the diffuser has no base
granularity such as that found in ground glass, since this matrix
will be captured in the image. A pity since ground glass has
little or no effect on colour balance that some of the other
diffusing screens seem to have. Flashed opal diffusers are very
good though costly, but they can, like the plastic diffusers be
placed against the emulsion side if necessary for maximum effect
when imaging the very worst examples of film in poor condition.
Too intimate a contact can raise Newton Rings which will be
counterproductive. Yet even at a distance of about 10-20mm from
the emulsion plane, where the plastic diffuser in slide copiers
is commonly sited, the suppression of unwanted artifacts is still
satisfyingly efficient.

In the
worst case scenario of film condition, it maybe necessary to
employ a wet mounting of the film between glass slides. This can
nullify the major marking/scratches by cancellation of refraction
that will occur in the angular interfacing in these grooves in
the acetate film base. It is a messy process and not easily
contrived to remain bubble free for the duration. However for 99%
of our aging nostalgic slide and B&W negative collections,
the dry diffusion screen will no doubt make a significant
difference to their presentation. In the final analysis personal
preferences reign regardless.

Diffusion
Screens and The Film Scanner

The effects of
diffused light extend to the 35mm dedicated film scanner too as
illustrated below. Again unsized crops from B&W film around
45 years old, but scanned in a Minolta Dual Scan 111. A flashed
Opal diffuser was used here, placed against the film's emulsion
layer where the effects of Newton Ring generation were
considerably reduced compared to the potential generation of NR
raised if the opal glass was positioned against the film's glossy
base layer. Again the differences between the images are clearly
obvious. If any softening of the image has occurred by the
process then it is very subtle and from my perspective welcome in
reducing the coarseness of the silver grain very slightly. It
appears too that the image's contrast has been somewhat reduced
:-

The 'cleansed'
appearance of the resulting image can fool the subconscious mind
into believing that there has been a softening of detail simply
because the acuity of the grain structure is somewhat reduced
along with the removal of fine scratches and the like.
Conversely, from another perspective, the presence of the sharp
representations of the unwanted detritus on the normal non
diffused lit image can trick the subconscious into extolling the
virtues of the original taking lens, which of course played no
part in their appearance ! I think there is much psychology
involved in the appraising of imagery.

Last
Word

The perceived
improvements from this simple diffuser technique are elevated in
part, by the entirely subjective viewpoint we all hold concerning
imagery, and most especially of the human face. I suspect a
favourite photograph is what we want it to be, and little else.
The humble diffusion screen, often maligned in microscopy, helps
in part to achieve just that.